It is well known to react reaction of saturated linear fatty acid chlorides with linear tertiary amines in a variety of inert solvents to make disubstituted ethanones, such as alkyl ketene dimers, which are useful as constituents of paper sizes as well as for other applications in industry. For instance, U.S. Pat. Nos. 2,238,826 and 2,369,919, and an article by J. C. Sauer, Ketene Dimers from Acid Halides, in the Journal of the American Chemical Society, 69 2444-8 (1947) describe such reactions. Among the known useful solvents are benzene, toluene, xylene, ligroin, chlorobenzene, dichlorobenzene, diethyl ether, dibutyl ether, chloroform, carbon tetrachloride, and trichloroethylene. The known families of solvents can be classified as:
1. alkanes and alkane petroleum fractions; PA1 2. cycloalkanes such as cyclohexane and methylcyclohexane; PA1 3. aromatic hydrocarbons such as benzene, toluene, and xylene; PA1 4. chlorinated solvents such as chlorobenzene, dichlorobenzene, chloroform, carbon tetrachloride, and trichloroethylene; and PA1 5. ethers such as diethyl ether.
The reaction in the said solvents to produce alkyl ketene dimer (AKD) in solution also produces linear tertiary amine hydrochloride (LTEA-HCl) as a finely divided precipitate. For some of the known solvent families, problems encountered in the alkyl ketene dimerization reaction are low AKD selectivity (&lt;80%), an excessively viscous dimerization mixture, or very small amine-hydrochloride crystals in the dimerization slurry that are hard to separate by mechanical procedures such as filtration and/or centrifugation and decantation.
U.S. Pat. Nos. 2,238,826 and 2,369,919 indicate that saturated acyclic amines free of active hydrogen are preferred for the said reaction, although they indicate that suitable amines include 1-methylpiperidine, 1-isopropylpiperidine, and 1-methylpyrrolidene. From the disclosure, of the '919 patent it is apparent that the use of the preferred linear amines requires a very dilute reaction mixture (100-200 parts solvent per tenth mol of each reactant to obtain a yield of 60 to 97%. That degree of dilution appears to be necessary to prevent the dimerization reaction slurry from becoming highly viscous, which would make separation of a linear tertiary amine hydrochloride (LTEA-HCl) precipitate from the reaction mixture extremely difficult and impossible by filtration as disclosed in the patent.
German OLS 29 27 118 discloses a process to make diketenes using of a mixture of amines, which includes trimethylamine and at least one other tertiary amine. The trimethylamine is said to avoid the high viscosity in the reaction mixtures with fatty acid halides that higher-substituted trialkylamines were found to cause. The other amine or amines may include 1-methylpiperidine or 1-methylpyrrolidene. The solvents used include alkanes and cyclo alkanes, as well as aromatic hydrocarbons, the reaction temperature is between 60.degree. and 90.degree. C., and the post-reaction heating is at 80.degree.. Separation of LTEA-HCl precipitate from the dimerization slurry involves liquid/liquid water extraction of the LTEA-HCl salt, which introduces moisture that is likely to cause hydrolysis of the alkyl ketene dimer and lessening of product yield. Yields of 87 to 92% are obtained and presumably cannot be improved further because of the inherent nature of the reaction.
There is a need for an alkyl ketene dimerization process that avoids excessive viscosity and small amine-hydrochloride crystals in the dimerization slurry, to facilitate the separation of amine-hydrochloride precipitates in consistently high yields by such mechanical separation methods as filtration, sedimentation and decantation, and without using uneconomic volumes of solvent, as disclosed in U.S. Pat. No. 2,369,919.
Also, because the conventional dimerization solvents used in present industrial applications are either unacceptably toxic, particularly benzene, diethyl ether and toluene, used in the said U.S. Pat. No. 2,369,919), or are known to be environmentally objectionable, including the whole class of the chlorinated solvents. There is therefore a need for a alkyl ketene dimerization process that uses a solvent with less health and environmental problems than aromatic hydrocarbons, such as benzene and toluene or halogenated solvents as used in the German OLS.